Natural resources such as gas, oil, and water residing in a geological formation may be recovered by drilling a wellbore into the formation while circulating a drilling fluid in the wellbore. After terminating the circulation of the drilling fluid, a string of pipe (e.g., casing) is run into the wellbore in order to provide structural support for the wellbore sides. The casing may be metal (e.g., steel).
Primary cementing may be performed whereby a cement slurry is injected into the annulus between the casing and the geological formation. The cement is permitted to set into a hard mass (i.e., a sheath) to thereby support the string of pipe within the wellbore and seal the annulus.
To determine the condition of the hardened cement, an evaluation log was introduced in the 1960s, using sonic attenuation measurements. Through years of evolution, acoustic solutions now include sonic attenuation measurements with borehole compensation and ultrasonic impedance imaging with azimuthal sensitivity. These evaluation technologies are being challenged to meet new environmental conditions (such as wellbores located in deep water) and new regulatory requirements. The acoustic impedance contrasts between heavy drilling mud and light cement, for example, may be close to or beyond the measurement sensitivity limitations of existing sensors, while heavy casings present an additional technical barrier for traditional acoustic solutions.